CT filters are used in a variety of direct signal processing applications where high speed and/or low power dissipation are needed. An example of an appropriate application of CT filters in direct signal processing is the read channel of a disk drive whose requirements for speed and power are such that CT filters are preferred. Other applications for CT filters include receivers, which receive data from local area networks or high speed data links, and wireless communication systems.
One type of CT filter is known as a G.sub.m -C filter. Such filters are discussed in detail in Tsividis "Integrated Continuous-Time Filter Design: an Overview" IEEE Journal of Solid State Circuits, Vol. 29, No. 3, March 1994, and Schaumann et al. "Design of Analog Filters" Prentice Hall, Englewood Cliffs, N.J., pp 457-485, the contents of which are incorporated herein by reference. The basic building block of a G.sub.m -C filter is a transconductance amplifier comprising a transconductor element and a capacitor or a pair of capacitors. A transconductor is an element whose output current is linearly related to an input voltage by a transconductance G.sub.m. For example, I.sub.o =G.sub.m.multidot. 2V.sub.i, where I.sub.o is the output current and V.sub.i is the input voltage.
An example of a transconductance amplifier integrator 10 is shown in FIG. 1. The amplifier integrator 10 of FIG. 1 comprises the transconductor 12 and the capacitor 14. As indicated above, the output current of the transconductor I.sub.o is equal to G.sub.m.multidot. 2V.sub.i. The output voltage V.sub.o is given by V.sub.o =V.sub.i G.sub.m /sC where C is the capacitance of the capacitor 14 and s is the complex frequency.
The frequency response of the transconductance amplifier integrator 10 is determined by the control voltage V.sub.c which is received as an input 13 in the transconductor 12. The control voltage is used to compensate for transconductor and capacitor variations (e.g., power supply variations, temperature variations, and integrated circuit process parameter variations). These variations are common in FET integrated circuits used to implement G.sub.m -C filters.
A transconductor 12 implemented using FETs is shown in FIG. 2. The transconductor 12 comprises two legs 21 and 23. The two legs 21 and 23 have fixed equal current sources 22 and 24. The input voltages +V.sub.i and -V.sub.i are applied to the gates of the FETs 26 and 28. The control voltage V.sub.c is applied to the degeneration FET 30. The voltage V.sub.Q is a quiescent voltage. The output current I.sub.o flows in the two FETs 26 and 28 in opposite directions.
An integrated circuit G.sub.m -C filter implemented using a plurality of transconductance amplifiers is shown in FIG. 3. The filter 40 of FIG. 3 comprises four transconductors 42, 44, 46, 48 and two capacitors 47 and 49. The filter 40 has a low pass output V.sub.LP and a bandpass output V.sub.BP.
Typically, the control voltage V.sub.c for the transconductors in a G.sub.m -C filter is generated by a phase-locked-loop (PLL). A circuit 60 in which an exemplary phase-locked-loop is used to generate the control voltage V.sub.c is shown in FIG. 4. As shown in FIG. 4, the control voltage for the G.sub.m -C filter 40 is generated using the PLL 50. The PLL 50 comprises a phase comparator 52 which receives an external reference signal such as a signal 51 from a crystal oscillator (not shown) and a signal 53 from a voltage-controlled-oscillator (VCO) 54. The phase comparator 52 outputs a frequency incrementing signal FUP and a frequency decrementing signal FDN to a charge pump 56. The FUP signal is generated when the VCO output signal lags the crystal oscillator signal. The FDN signal is generated when the VCO output signal leads the crystal oscillator signal.
Depending on which of the FUP or FDN signals is applied, the charge pump 56 will generate positive or negative current pulses which add to or subtract from the total charge accumulated in a charge accumulating device (e.g. capacitor) 58. The device 58 generates a voltage V.sub.c which is dependent on the total charge in the accumulating device 58. This voltage V.sub.c is the input voltage to the VCO 54. If V.sub.c increases or decreases, the output frequency of the VCO increases or decreases. The voltage V.sub.c also serves as the control voltage for the transconductors in the G.sub.m -C filter 40 and is used to compensate the transconductors for a variety of parameter variations. Phase-locked-loops of the type described above are disclosed in U.S. Pat. No. 5,126,692 the contents of which are incorporated herein by reference. It should be noted that the VCO 54 may implemented by a voltage-to-current convertor and a current controlled oscillator (ICO).
In general, the VCO 54 is made using the same basic transconductor and capacitor elements as the G.sub.m -C filter 40. The same quantity V.sub.c used to control these elements in the VCO 54 is used to control the corresponding elements in the main G.sub.m -C filter 40.
However, the above described technique for controlling a G.sub.m -C filter 40 is limited by the effective control range of the transconductor elements. If the control range is not sufficient, then the gain of the transconductors (I.sub.o /V.sub.i) cannot be changed enough to account for all of the possible parameter variations and the filter frequency response will not be correct.
Accordingly, it is an object of the invention to increase the control range of the transconductor elements of a G.sub.m -C filter. This makes it possible to maintain the filter frequency response over an increased range of transconductor and capacitor variations.